The present invention relates to a porous carbon material and a method of producing the same, and an electrical double layer capacitor using the porous carbon material. Particularly, the present invention provides a porous carbon material which has a large specific surface area per volume (referred to as a xe2x80x9cvolume specific surface areaxe2x80x9d hereinafter), and which significantly increases in capacitance per unit volume when used as a polarizable electrode material, and an industrial method of producing the porous carbon material in high yield and at low cost. Also, the present invention provides an electrical double layer capacitor using the porous carbon material and having a large capacitance even with a small size.
In an interface between two different phases including a solid electrode and an electrolytic solution, positive and negative charges are arranged and distributed with a very short distance therebetween. For example, with the positively charged electrode, anions in the solution are arranged for supplement for the charge. A layer caused by the charge arrangement is referred to as an xe2x80x9celectrical double layerxe2x80x9d. The electrical double layer is formed by non-Faradaic reaction without electron transfer between the electrode and the ions. The capacitance expressed at the electrode interface accompanying the formation of the electrical double layer is referred to as xe2x80x9celectrical double layer capacitancexe2x80x9d, and an energy storage device utilizing the capacitance is referred to as an xe2x80x9celectrical double layer capacitorxe2x80x9d.
The electrical double layer capacitor has the properties that it has excellent instantaneous charge-discharge performance, and causes less deterioration in charge-discahrge cycle life performance. Therefore, the electrical double layer capacitor is useful as a backup power supply for an electronic apparatus such as a microcomputer comprising IC memory, and a power supply for an electric vehicle.
In the electrical double layer capacitor having a structure in which an electrolyte is held between a pair of polarizable electrodes, stored capacitance C is represented by the following equation (1):
C=∫[xcex5/(4xcfx80xcex4)]dsxe2x80x83xe2x80x83(1)
(wherein xcex5: dielectric constant of electrolytic solution , xcex4: distance between the electrode surface and the ion center, s: surface area of the electrode interface.)
Therefore, by using a polarizable electrode material having a large specific surface area, an electrical double layer capacitor having a large capacitance can be obtained.
A known material with a large specific surface area which can be used as the polarizable electrode is activated carbon. The activated carbon is generally produced by an oxidizing gas activation method comprising oxidizing a waste resin, a pulp production residue, coal, coal cokes, wood, coconut shell, or the like, which is used as a raw material, with stream, air, oxygen, CO2, or the like to form pores.
Since the oxidizing gas activation method uses no special chemical and has no problem of equipment corrosion, the method is advantageous from the viewpoint of cost. However, it is known that activated carbon having a, large specific surface area cannot be efficiently obtained by oxidizing gas activation. For example, in an attempt to obtain activated carbon having a surface area per unit weight (referred to as a xe2x80x9cweight specific surface areaxe2x80x9d hereinafter) of as large as about 1500 to 3000 (m2/g), which is required for the polarizable electrode material, the yield is as low as 20% or less.
On the other hand, a chemical activation method of forming pores by using a chemical is also known. Particularly, an alkali activation method using alkali can obtain activated carbon having a surface area of 1500 to 3000 (m2/g) per unit weight in a yield of 60% or more. The alkali activation method can also increase the specific surface area by increasing the amount of the alkali used. For example, by using alkali in an amount of about 3 to 5 times (mass ratio) as large as the raw material, a specific surface area of 2000 m2/g or more can be obtained.
In recent years, electronic apparatus, electric vehicles, etc. have been required to be decreased in weight and size, and the energy density of the carbon material used for these applications has been increasingly required to be increased.
In an attempt to decrease the weight and size of the electrical double layer capacitor, therefore, it is required to increase not only the capacitance per unit mass (referred to as the xe2x80x9cweight specific capacitancexe2x80x9d hereinafter), but also the capacitance per unit volume (referred to as the xe2x80x9cvolume specific capacitancexe2x80x9d hereinafter). More specifically, in application of the electrical double layer capacitor, the target value of the capacitance per unit volume is as high as 20 F/cm3 or more. The capacitance per unit volume (F/cm3) is obtained by multiplying the capacitance per unit mass (F/g) by a packing density.
The above equation (1) indicates that the capacitance per unit mass (F/g) increases as the surface area per unit mass increases. However, in fact, the capacitance per unit volume (F/cm3) is liable to be saturated when the surface area per unit weight is about 2000 (m2/g), and inversely decreased when the surface area per unit weight exceeds about 2500 (m2/g). Therefore, the target value of the capacitance per unit volume of 20 F/cm3 or more cannot be easily achieved by the conventional method of increasing the specific surface area.
This is possibly mainly due to the fact that the packing density is decreased by increasing the specific surface area.
Another conventional method for improving the capacitance per unit volume has been proposed.
Namely, it has been proposed to secure a high packing density by using meso carbon micro beads as an alkali activation raw material (Application No. 2634658). However, it is disclosed that by the alkali activation method using meso carbon micro beads having smooth surfaces, a high packing density of 0.90 g/cm3 can be achieved with a surface area per unit weight of 470 (m2/g), but when the surface area per unit weight is increased to 1000 (m2/g) or more by activation, the packing density can be increased to only about 0.50 g/cm3 at most. Furthermore, in an attempt to increase the surface area per unit weight to about 2000 (m2/g), only a packing density of as low as 0.46 g/cm3 can be obtained.
The alkali activation method is basically a method of producing micro pores having diameters of less than 2.0 nm to increase the specific surface area. For example, Japanese Unexamined Patent Publication No. 1-230414 discloses that in alkali activation of meso carbon micro beads under an inert atmosphere, the ratio of micro pores of less than 2.0 nm is 85% or more of the total pore volume. This publication also discloses in examples that in activation with potassium hydroxide in an amount of 3 to 5 times as large as the meso carbon micro beads, activated carbon having a weight specific surface area of 1500 to 3000 (m2/g) can be obtained. In this case, the ratio of the weight specific surface area of mesopores having diameters of 2.0 nm or more is only about 0.8 to 3.0% of the total specific surface area.
Pores of the polarizable electrode material for the electrical double layer capacitor, which are effective to adsorb and desorb an electrolyte, are generally mesopores of 2.0 nm or more. However, the conventional alkali activation method is difficult to produce many mesopores of 2.0 nm or more.
Although the alkali activation can easily secure a certain weight specific surface area, it requires a large amount of alkali, and has the problems of high chemical cost and equipment corrosion, thereby causing difficulties in industrial mass production.
Therefore, a method has been proposed, in which a metal or a salt thereof other than alkali metals is added in oxidizing gas activation for activating a carbon material with steam or the like to form mesopores.
For example, Yoshizawa et al. (Tanso, 181, 8-13 (1998)) disclose steam activation for forming mesopores in which acetylacetonato-complex salt of iron (III), nickel (II), cobalt (II) is added to coal. By this steam activation method, activated carbon supporting metal oxides can be obtained.
Japanese Unexamined Patent Publication Application No. 10-297912 discloses carbonization or activation in a weakly oxidizing atmosphere in which a transition metal or a compound of a transition metal such as Ti, Cr, Mn, Fe, Co, Ni, Cu, W, or the like is added to facilly graphitizable carbon such as cokes, or hardly graphitizable carbon such as phenol resin. In this method, the pore diameter distribution is maximized to obtain meso carbon. However, this publication discloses that although mesopores can be formed regardless of the type of the metal added, the specific surface area of cokes is less increased, and the specific surface area of the phenol resin is not so increased.
Japanese Unexamined Patent Publication Application No. 10-172870 teaches a method in which activated carbon, activated carbon fibers or carbon black is activated with steam together with a metal or metal compound to disperse fine particles of the metal or metal oxide, imparting conductivity.
The present invention has been achieved for solving the above-described problems, and an object of the present invention is to provide a porous carbon material which has a volume specific surface area of 1000 (m2/cm3) or more and which significantly increases in capacitance per unit volume when used as a polarizable electrode material, and an industrial method capable of producing the porous carbon material in high yield and at low cost. Another object of the present invention is to provide an electrical double layer capacitor using the porous carbon material and having a large capacitance even with a small size.
The inventors performed study about an industrial method capable of producing a porous carbon material suitable as an electrode material for a high-performance electrical double layer capacitor. As a result, it was found that a porous carbon material having a large volume specific surface area and significantly increasing in capacitance per unit volume when used as a polarizable electrode could be obtained by alkali activation using a soft carbon-type carbon material as a raw material in coexistence with specified metal ion and a carboxylic acid ion. It was also found that this method uses a smaller amount of alkali than a conventional method, and thus the problems of chemical cost and equipment corrosion can be resolved. These findings resulted in achievement of the present invention.
Namely, the present invention relates to a method of producing a porous carbon material comprising activating a soft carbon-type carbon material with alkali in the presence of a carboxylic acid ion and at least one metal ion selected from the group consisting of iron ions, cobalt ions, manganese ions and nickel ions.
In this production method, the soft carbon-type carbon material preferably comprises at least one selected from mesophase microspheres and bulk mesophases.
The metal ion is preferably supplied by a metal chloride.
The carboxylic acid ion is preferably at least one ion selected from an acetic acid ion, a citric acid ion, and a benzoic acid ion.
In the present invention, the porous carbon material is obtained by any one of the above production methods, and has a surface area per unit volume is 1000 m2/cm3 or more. The porous carbon material preferably has a packing density of 0.5 g/cm3 or more, which is obtained by a predetermined packing method.
Furthermore, the present invention provides an electrical double layer capacitor using the porous carbon material as a polarizable electrode material. The electrical double layer capacitor preferably has an capacitance per unit volume of 20 F/cm3 or more.